Phytoplankton Cell Size Reduction in Response to Warming Mediated by Nutrient Limitation

Shrinking of body size has been proposed as one of the universal responses of organisms to global climate warming. Using phytoplankton as an experimental model system has supported the negative effect of warming on body-size, but it remains controversial whether the size reduction under increasing temperatures is a direct temperature effect or an indirect effect mediated over changes in size selective grazing or enhanced nutrient limitation which should favor smaller cell-sizes. Here we present an experiment with a factorial combination of temperature and nutrient stress which shows that most of the temperature effects on phytoplankton cell size are mediated via nutrient stress. This was found both for community mean cell size and for the cell sizes of most species analyzed. At the highest level of nutrient stress, community mean cell size decreased by 46% per degrees C, while it decreased only by 4.7% at the lowest level of nutrient stress. Individual species showed qualitatively the same trend, but shrinkage per degrees C was smaller. Overall, our results support the hypothesis that temperature effects on cell size are to a great extent mediated by nutrient limitation. This effect is expected to be exacerbated under field conditions, where higher temperatures of the surface waters reduce the vertical nutrient transport

Effect of climate warming on phytoplankton size structure and species composition: an experimental approach

The study aimed to elucidate several of the mechanism which may induce phytoplankton size reduction in response to warming which approached through different experiments. The factors studied were direct temperature, gazing and nutrients limitation effects on phytoplankton size structure and species compositio

ANOVA of temperature and nutrient effects.

<p>Two-factor ANOVA of temperature and nutrient level effects on log¹⁰ cell volume (µm³) of the entire phytoplankton community and of the individual species arranged by size from the largest to the smallest; <i>N</i> = 27, except for <i>Ceratium tripos, Ceratium fusus</i>, and <i>Chaetoceros brevis</i> which disappeared from the N3 – 19.5°C treatment combination (<i>N</i> = 24).</p

Cell sizes of phytoplankton taxa decrease with temperature and increase with dilution rate

<p>(continued). Cell volume in µm³ (log¹⁰-scale) in response to temperature and nutrient regime; N1: 50% dilution three times per week; N2: 25% dilution three times per week; N3: no dilution. Species codes: GY: <i>Gymnodinium</i> sp.; LD: <i>Leptocylindrus danicus</i>; CHC: <i>Chaetoceros curvisetus</i>; PY: <i>Pyramimonas</i> sp.; CC: <i>Cylindrotheca closterium</i>; PC: picophytoplankton.</p

Particular matter C∶N ratios increase with tmeperature and decrease with dilution rate.

<p>Molar C∶N ratios of particulate, organic matter in response (log¹⁰-scale) to temperature and nutrient regime; N1: 50% dilution three times per week; N2: 25% dilution three times per week; N3: no dilution.</p

Two-factor ANOVA of temperature and grazing effects on total Biomass (B_tot) and community cell size (B_tot/N_tot).

<p>Two-factor ANOVA of temperature and grazing effects on total Biomass (B_tot) and community cell size (B_tot/N_tot).</p

Two-factor ANOVA of temperature and grazing effects on cell sizes.

<p>Two-factor ANOVA of temperature and grazing effects on cell sizes.</p

Multiple Regression of cell volume on temperature and C∶N ratios.

<p>Regression according to the model log¹⁰ V = a+b.t+c.log¹⁰(C∶N), where t is expressed in °C and V in µm^3C; partial correlation coefficients (<i>R_t, R_CN</i>), partial probabilities of error (<i>P_t, P_CN</i>), <i>R²</i> for the full model, and probability of error for the full model (<i>P_model</i>); <i>N</i> = 9, except for <i>Ceratium tripos, Ceratium fusus</i>, and <i>Chaetoceros brevis</i> which disappeared from the N3 – 19.5°C treatment combination (<i>N</i> = 6). The temperature effect is also shown as % volume reduction per °C.</p

Phytoplankton community mean cell size decreases with temperature and increases with dilution rate.

<p>Community mean cell volume in µm³ (log¹⁰-scale) in response to temperature and nutrient regime; N1: 50% dilution three times per week; N2: 25% dilution three times per week; N3: no dilution.</p

Regressions (model: y = ax+b) of log¹⁰ total biomass (B_tot) and Community cell size (B_tot/Nt_ot) on log¹⁰ temperature (°C) for the different species and grazing regimes.

<p>Regressions (model: y = ax+b) of log¹⁰ total biomass (B_tot) and Community cell size (B_tot/Nt_ot) on log¹⁰ temperature (°C) for the different species and grazing regimes.</p